1. Preheat the oven to 375°F. Line large baking sheets with parchment paper.

2. Sift the flour, cocoa powder, baking soda, and salt into a medium bowl.

3. In the bowl of an electric mixer, beat the butter and brown sugar on low until combined, then on medium-high for 3 minutes until fluffy. Scrape the bowl. Add egg and vanilla; beat for another 3 minutes.

4. Add half the flour mixture and half the buttermilk; beat on low until combined. Repeat with remaining flour and buttermilk. Batter will be thick and fluffy.

5. Use a medium scoop to drop mounds of batter onto baking sheets, spacing 2 inches apart. Bake for 11 minutes, or until tops are dry and spring back when gently pressed. Cool on pans for 5 minutes, then transfer to wire rack to cool completely.

6. To make the filling, beat powdered sugar and butter on low until blended, then on medium for 3 minutes until smooth. Scrape the bowl. Add salt, peppermint extract, and cream. Beat on medium for 1 minute until light and creamy. Add green gel food coloring if using.

7. Use a scoop or piping bag to dollop filling onto half the whoopie pies. Top with remaining cakes to form sandwiches.

8. Store at room temperature for 1 day or refrigerated for up to 4 days. Best served at room temperature. Chill in fridge for 1 hour before transporting to help firm them up.